A long term evolution (LTE) Rel-8/9/10 communications system employs a dynamic scheduling technology to improve system performance, that is, an evolved NodeB (eNB) schedules and allocates resources according to a channel state of each user equipment (UE), so that each scheduled user performs communication on an optimal channel of the user. In downlink transmission, the eNB sends a physical downlink shared channel (PDSCH) and a physical downlink control channel (PDCCH) corresponding to the PDSCH to each scheduled UE according to a result of the dynamic scheduling, where the PDSCH bears data that the eNB sends to the UE, and accordingly, the PDCCH is mainly used for indicating a transmission format or scheduling information of the PDSCH, for example, resource allocation, a transport block size, a modulation and coding scheme a transmission rank, precoding matrix information, and so on.
In one subframe, all PDCCHs used for uplink and downlink scheduling are multiplexed on N control channel elements (CCE) in a PDCCH region, where N is greater than 1, and the control channel elements are numbered from 0. Each PDCCH is an aggregation of L consecutive CCEs, where L is one of 1, 2, 4, or 8, that is, the PDCCH has four aggregation levels in total. The number of CCEs aggregated in each PDCCH is determined by the size of an information block size in the PDCCH and a channel state of a UE corresponding to the PDCCH. Before the PDCCH is sent, the N CCEs multiplexed in the PDCCH region are interleaved, and then the interleaved CCEs are mapped to a reserved RE in the PDCCH region in sequence and sent.
At a receiving end, the UE needs to perform blind detection on the N CCEs to obtain a PDCCH corresponding to the UE. At each CCE aggregation level, PDCCH candidates are limited. The less the candidate PDCCHs are, the less the number of blind detection times of the UE is. For example, in the prior art, when the CCE aggregation level L is equal to 8, the number of PDCCH candidates is 2, that is, only CCE 0 to CCE 7 and CCE 8 to CCE 15 need to be detected. Although such CCE assignment principle can reduce the number of blind detection times, the number of blind detection times corresponding to each aggregation level is still in positive correlation with the number N of CCEs in the PDCCH region, that is, the number of blind detection times increases as the N increases. To further reduce the complexity of blind detection, at each CCE aggregation level, the maximum number of times of blind detection that the UE needs to perform is defined, which is called a search space. Search spaces are classified into a common search space and a UE-specific search space, and the difference between the two lies in that a location of a start CCE in the common search space is fixed while a start CCE in the UE-specific search space is determined by an identifier of the UE and a subframe number of a subframe where the PDCCH is located. The common search space and the UE-specific search space may overlap each other.
An existing PDCCH is enhanced in LTE Rel-11, that is, a part of resources in a PDSCH region are divided to transmit an enhanced physical downlink control channel (EPDCCH), so that resources assigned to the control channel are more flexible, and are no longer limited by three orthogonal frequency division multiplexing (OFDM) symbols. The EPDCCH may use a transmission manner based on a demodulation reference signal (DMRS) to implement spatial reuse, so as to improve transmission efficiency of the control channel. For example, control channels of UEs serving different radio remote units (RRU) may occupy the same time frequency resource as long as being desirably isolated in space, and in this way, the capacity of the PDCCH or the number of UEs scheduled at the same time is improved.
Main conclusions passed on the the 3rd generation partnership (3GPP) radio access network (RAN) 1 70bis standard conference are as follows. A UE performs blind detection in K EPDCCH sets, each EPDCCH set in the K EPDCCH sets is formed by M physical resource block pairs, and the value of M is 2, 4, or 8. In a case of a normal subframe (normal cyclic prefix) or special subframe (normal cyclic prefix) ratio 3, 4, or 8, when the number of valid resource units included in each physical resource block pair is less than a predetermined threshold, aggregation levels that can be supported by the EPDCCH are 2, 4, 8 or 16; and in other cases, aggregation levels that can be supported by the EPDCCH are 1, 2, 4, 8, or 16.
The total number of blind detection times of the UE is 32 (in a special case such as multiple-input multiple-output (MIMO), the total number of blind detection times of the UE is 48). First, the number of blind detection times is assigned to the aggregation levels that can be supported by the EPDCCH, and then is assigned among EPDCCH sets corresponding to each aggregation level.
Transmission formats that can be supported by the EPDCCH mainly include downlink control information (DCI) format series 1X, including 1, 1A, 1B, 1C, and the like; DCI format series 2X, including 2, 2A, 2B, 2C, and the like; and DCI formats 0, 4, and the like used for indicating a data transmission format of an uplink traffic channel. A payload of the DCI format series 2X is generally much greater than that of the DCI format series 1X.
In the current standard, aggregation levels that can be supported by an EPDCCH are determined by the comparison between the number of valid resource units included in each physical resource block pair in a search space where the EPDCCH is located and a predetermined threshold. When the number of valid resource units included in each physical resource block pair is greater than the predetermined threshold, a transmission code rate of the EPDCCH transmitted in the DCI format 1A is not greater than 0.8, but this conclusion is not applicable to an EPDCCH transmitted in the DCI format series 2X. For example, if it is determined according to the predetermined threshold that aggregation levels that can be supported by the EPDCCH are 1, 2, 4, 8, and 16, when the EPDCCH is transmitted in the DCI format 1A and at the lowest aggregation level 1, the transmission code rate of the EPDCCH is not greater than 0.8. However, when the EPDCCH is transmitted in the DCI format series 2X and at the lowest aggregation level 1, it cannot be ensured that the transmission code rate thereof is within a certain threshold, and the transmission code rate thereof is even possibly greater than 1.
In one subframe, when aggregation levels that can be supported by an EPDCCH is determined according to the foregoing predetermined threshold, the determined lowest aggregation level may not support data transmission in the DCI format series 2X. In this case, the UE skips blind detection for the DCI format series 2X at the lowest aggregation level, and only detects EPDCCH candidates transmitted in the DCI format series 2X at other aggregation levels. With further consideration, in some overhead combinations, sizes of the control channel elements are not balanced, and sizes of EPDCCH candidates corresponding to a certain aggregation level are not balanced either, which may lead to a phenomenon that at the same aggregation level, some EPDCCH candidates support transmission in the DCI format series 2X, while some EPDCCH candidates do not support transmission in the DCI format series 2X. In this case, in the prior art, the UE also skips the EPDCCH candidates that do not support the transmission in the DCI format series 2X, which decreases the utilization of the number of EPDCCH candidates and the number of blind detection times.